Film bulk acoustic resonators (FBARs) are devices that include a thin film piezoelectric material sandwiched between two electrodes and acoustically de-coupled from the surrounding medium. These devices can be made to be strongly resonant with narrow bandwidth at high frequencies (several gigahertz), consistent with wireless communication frequencies. Many applications exist for FBARs including filters, duplexers, etc.
The resonant frequency of an FBAR depends on many factors, including the deposition thickness of the thin film that makes up the resonator. This makes the resonant frequency of FBARs dependent on manufacturing variations. FBARs on the same integrated circuit (and in the same locale on the same integrated circuit) may be well matched because of the locality of deposition methods, but absolute resonant frequency values tend to vary from chip to chip and/or batch to batch.